The sucking lice, or Anoplura (Order Phthiraptera, Suborder Anoplura), are parasitic insects found on nearly all groups of mammals. Of the 15 recognized families of Anoplura, two families, Pediculidae and Pthiridae, have species which infest humans. These human lice are: the head louse, Pediculus humanus capitis; the body or clothing louse, Pediculus humanus humanus, sometimes called Pediculus corporis; and the crab louse, Pthirus pubis. These human lice have bodies that are constructed of a hard chitinous exoskeleton, like all insects, and are very small (about 2-3 mm in length). The human lice are prolific egg (ova) layers, and they typically deposit eggs either on a hair or fabric fiber and attach them firmly with a cement-like excretion. Nymphs hatch from the eggs in about six to ten days, depending on temperature. The empty shells remaining after the nymphs emerge from the eggs look like white grains of sand, and these shells are called nits.
Infestation of the human body by lice is an increasingly prevalent social and health problem in many countries, including the United States (US). Head lice affect as many as 6-12 million people each year globally. The problem is particularly prevalent in preschool and elementary-age children (aged 3-10) and their families. Head lice infestation typically causes itching of the scalp. As the lice feed on human blood, they may cause lesions to develop on the scalp, swollen glands on the neck or under arms, or other symptoms. Head lice infestation also causes serious problems due to the negative social implications of the infestation. Head lice infestations are believed to account for 12 million to 24 million missed school days a year in the US. A majority of US public schools have instituted a no-lice, or a no-lice and no-nit policy, forcing absenteeism on children and requiring working parents to stay at home to look after them or provide other arrangements for child care. For example, see Burkhart and Burkhart (2006, Expert Opin. Drug Saf. 5(1):169-179) for a review.
Body lice (Pediculus humanus humanus and Pthirus pubis) are also troublesome for humans. In addition to the problems caused by head lice, body lice carry the additional hazard of being the vectors of certain diseases, such as exanthematic or epidemic typhus and recurrent fever.
The louse's hard chitinous exoskeleton serves as protection from dehydration and external elements and also resists insecticides and other treatments, making control difficult. Additionally, lice eggs are protected by a surrounding hard keratin sheath. Although lice may be more readily affected by the use of an insecticide, the eggs often remain resistant to such attack. Thus, treatment of a lice infestation may necessitate both a pediculicide, which kills the adult lice, and an ovicide, which interrupts the development of the eggs.
Biologically active agents, such as avermectins (such as ivermectin), organophosphates (such as malathion), organochlorines (such as lindane and gammabenzene hexachloride), pyrethrins, and synthetic pyrethroids (such as permethrin), have been used for some time in attempts to control lice. Each of these agents, however, has drawbacks. For example, lindane has a relatively poor safety profile. Natural pyrethrin requires frequent follow-up treatments because it provides only short-term residual action. Synthetic pyrethroids, although more effective against lice than natural pediculicides, are often more toxic to the subject being treated. In addition, resistance is an issue with such agents. Newer recent classes such as avermectins (such as ivermectin) are as yet unproven in their head lice control effectiveness in patients.
Pyrethrins are any one of six naturally occurring insecticides extracted from the chrysanthemum flower. Along with its synthetic derivative, permethrin, these molecules act on susceptible head lice by increasing sodium levels in the nervous system of the lice. The increased sodium levels cause membrane depolarization in the nervous system, which eventually leads to spastic paralysis and death of the head lice. When first introduced, both pyrethrin and permethrin were highly effective at eliminating susceptible lice. In the late 1980s, various formulations of both active ingredients had a high efficacy for eliminating adult head lice and their nits. However, recent reports indicate that treatment-resistant strains of head lice have evolved for popular commercial products including NIX® (INSIGHT Pharmaceuticals, LLC., Tarrytown, N.Y.), which has 1% permethrin as an active ingredient, and various RID® (Bayer HealthCare LLC, Pittsburgh, Pa.) products, which have approximately 0.33% pyrethrin as an active ingredient. Given the prevalence and popularity of these products, it comes as no surprise that strains of treatment-resistant head lice have been identified in both the US and Europe due to the similar killing pathway for both insecticides.
Human lice reinfestation can occur via materials or certain surfaces that come into contact with humans, such as clothing, bedding, couches, chairs, blankets, and other fabric-based accessories such as hats, stuffed animals, towels, throw pillows, and the like. While mature adult lice may not be able to survive without a host longer than two days or so, eggs (which take around seven days to hatch) may be firmly cemented to clothing, fabric, bedding, and other fabrics that could contact human skin. Currently known methods of combatting such reinfestation include subjecting clothing, bedding, and fabrics to certain treatments, including exposing the textiles to excessive heat in a dryer, and placing the fabrics in plastic bags to which a vacuum can be applied to create a type of anaerobic environment that suffocates and even desiccates the lice. However, it is not always possible to use these methods on larger items, such as couches or car seats, and in some cases performing these methods may cross-contaminate other non-target items with lice.